Piunikaweb, which was first to report the issue
in January, contacted me with several concerning reports which I have subsequently been able to verify. Let’s break them down:

Apple

Apple's iPhone X (left) is having trouble being a phone

1. iPhone X Calls Freezing

While Apple is “looking into” reports of the iPhone X being unable to answer calls due to the phone’s display not waking up, now owners are finding they cannot hang up calls either (1,
2,
3,
4,
5,
6,
7,
8,
9, etc).

The problem seems to be both sensor and software related. Piunikaweb reports for some the phone display doesn’t come back on when moving the iPhone X away from their ear, while for others using hands-free calling the red hangup button is simply unresponsive.

2. The 911 Side Effect

The unwanted side effect of not being able to hang up calls on your iPhone X is the phone’s potential to
butt call emergency services
(simply hold the wake/lock button and a volume button for a few seconds). So affected users accidentally who do this are finding themselves blocking emergency numbers in their home country.

As you’d expect some of the stories make for painful reading (for example
1,
2,
3,
4,
5,
6,
7), such as:

“So my iPhone X just had a complete fit and dialled 112 emergency services! Cue “I’m so sorry....please hang up the call as I can’t” -
source, and “So I bought the iPhone X today and within the 1st hour called 911 twice and couldn’t hang it up” -
source

Rounding things off is a less serious third call-related bug which stops iPhone X owners from quitting apps while they are in a call (1,
2,
3,
4,
5). The official @AppleSupport service on Twitter is discussing this with users, but its policy of doing so privately (1,
2,
3) means no subsequent information has emerged.

Ultimately all these call issues suggest we can expect a dedicated iOS 11.2.6 bug fix within the next week. Or, if it takes longer, patches to be folded into the
essential iOS 11.3 update
which Apple has already promised will arrive before the end of the month.